Long-term objectives: analysis of the nature and ionic requirements of the mechanisms that regulate intracellular pH (pHi). Specific aims: (1) investigation of the mechanism by which 1- methyl, 3-isobutylxanthine (IBMX), an inhibitor of cyclic nucleotide phosphodiesterase, stimulates, in frog skeletal muscle, recovery of pHi from imposed acidification; (2) kinetic analysis of the mechanism by which the transporter in giant barnacle muscle removes acid equivalents from the fiber in response to acidification. General methods: electrophysiological methods, intracellular pH- sensitive microelectrodes.